R-290 PT Chart: How to Actually Read the Pressures

At 40 °F saturation, R-290 reads about 64 psig; at 100 °F saturation, about 174 psig. R-290 is propane, a single compound with no temperature glide, so its PT chart needs only one column. Use suction pressure for superheat and liquid-line pressure for subcooling. On the small sealed systems that use it, you charge by weight, not by gauge.

Most technicians meet R-290 in a reach-in cooler, an ice machine or a new household refrigerator. It catches people out in two ways. The pressures sit close to R-22, so it looks familiar on the gauges, but the charge is measured in ounces, so a gauge reading tells you far less than you are used to. And the Section 608 venting exemption for propane is real but narrow. It applies to named equipment, not to propane in general.

What are the actual R-290 pressures?

These values come from the National Refrigerants R-290 pressure-temperature chart. Pressures are in psig at saturation.

Temperature	R-290	Temperature	R-290
−40 °F	1.4 psig	50 °F	77.6 psig
−30 °F	5.7 psig	60 °F	93.0 psig
−20 °F	10.7 psig	70 °F	110.2 psig
−10 °F	16.7 psig	80 °F	129.3 psig
0 °F	23.7 psig	90 °F	150.5 psig
10 °F	31.8 psig	100 °F	173.9 psig
20 °F	41.1 psig	110 °F	199.6 psig
30 °F	51.8 psig	120 °F	227.8 psig
40 °F	63.9 psig	130 °F	258.7 psig

Propane stays above atmospheric pressure down to about −40 °F, where it reads just 1.4 psig. That is why it works in low-temperature cases without pulling the low side into a vacuum under normal conditions.

How does R-290 compare to R-22 and R-134a?

R-290 runs close to R-22 and far above R-134a, the refrigerant it most often replaces in small commercial equipment.

Saturation temperature	R-290	R-22	R-134a
40 °F	63.9 psig	68.6 psig	35.0 psig
100 °F	173.9 psig	195.9 psig	124.2 psig

The R-22 figures come from the Chemours guide used in our R-22 PT chart, and the R-134a figures from the Arkema chart used in our R-134a pressure chart. The takeaway is simple. If you read an R-290 cooler against an R-134a column out of habit, a 40 °F coil reading 64 psig will look like a coil at roughly 65 °F, about 25 degrees warmer than it is. Set the manifold or app to R-290 before you connect.

Why does an R-290 chart only need one column?

Because R-290 is a pure substance. A single compound boils and condenses at one temperature for a given pressure, so the bubble point and the dew point are the same number. There is no glide to correct for, unlike a zeotropic blend such as R-404A or R-448A, whose charts print two values per temperature.

Two practical consequences follow. A leak cannot change the composition of what is left in the system. And the saturation temperature you read off the chart is exact, whether you are on the suction line or the liquid line.

Is R-290 just propane?

Chemically, yes. R-290 is propane, the same molecule sold for grills and heaters. The difference is purity. Refrigerant-grade R-290 is supplied to a tight specification, while fuel propane is sold to a looser one and can carry other hydrocarbons and moisture. Neither belongs in a system that did not ship with R-290.

Propane is rated A3 by ASHRAE: lower toxicity, higher flammability. EPA lists its global warming potential at 3, which is why it keeps winning new end uses as HFCs are phased down. For how the A3 class sits next to A1 and A2L refrigerants, see types of refrigerant.

How should R-290 be charged?

By weight, to the charge printed on the nameplate. R-290 equipment is almost always a sealed, self-contained system with a small charge, often a few ounces per circuit. At that size, a fraction of an ounce moves the superheat noticeably, and the pressures can look acceptable across a wide range of charges.

In practice:

• Recover or remove the old charge according to the manufacturer's service instructions.
• Evacuate, then weigh in the full nameplate charge on a scale that reads small increments accurately.
• Use superheat and subcooling to confirm the system is working, not to top it off one burst at a time.

Pulling a deep vacuum before charging is good service practice. It is not a Section 608 evacuation requirement. The recovery levels in 40 CFR 82.156 are a separate rule about getting refrigerant out, not about dehydration. Superheat and subcooling covers what each reading actually tells you.

What does R-290 standing pressure tell you?

With the system off and equalized, the pressure equals the saturation pressure at the refrigerant's temperature, as long as some liquid remains. After a long rest that is close to room temperature: about 110 psig at 70 °F and about 129 psig at 80 °F.

That confirms propane is present. It does not confirm the charge is right, because a system missing half its few ounces still holds saturated liquid at the same pressure. A standing reading well below saturation for the room means the liquid is gone.

What does Section 608 actually say about R-290?

This is where competitors most often blur the line. Section 608 does not exempt propane. It exempts propane in specific end uses, listed at 40 CFR 82.154(a)(1)(viii):

Propane (R-290) is an exempt substitute in	Source
Stand-alone retail food refrigerators and freezers	82.154(a)(1)(viii)
Household refrigerators, freezers and combination units	82.154(a)(1)(viii)
Self-contained room air conditioners and heat pumps for residential and light commercial use	82.154(a)(1)(viii)
Vending machines, self-contained commercial ice machines, very low temperature equipment and water coolers	82.154(a)(1)(viii)
Refrigerated food processing and dispensing equipment, effective July 15, 2024	82.154(a)(1)(viii)

In those end uses, the venting prohibition in 82.154(a) does not apply to the propane. Outside them, R-290 is a non-exempt substitute. That means the venting prohibition, the required practices in 82.156 and the requirement in 82.154(b) to use recovery equipment certified for that refrigerant and appliance all apply.

Separately, EPA's SNAP program lists R-290 as acceptable only subject to use conditions in each end use. Those conditions include charge limits and required safety standards, and they govern how new equipment is built. They are not something you adjust in the field. The EPA SNAP listing for household refrigerators is one example.

The exemption is a regulatory one, not a safety one. Releasing even a few ounces of propane in a closed kitchen next to a pilot light is dangerous whether or not it is legal.

What this means on the job

Read the nameplate before you touch the gauges. If it says R-290, change how you work.

• Ignition sources first. Ventilate, keep sparks and flames away, and use tools and recovery equipment rated for flammable refrigerants. Refrigerant safety covers why.
• Charge by weight. The nameplate figure is the answer. Pressures only confirm it.
• Know which rule applies. A stand-alone reach-in is an exempt end use. Propane in an application EPA has not listed is not, and the full Subpart F rules follow it.
• Never substitute it. Putting propane into a system designed for R-134a or R-22 is not an approved retrofit.

Self-contained propane equipment is small-appliance work, which is the heart of Type I. If that is the section you are working toward, the Type I practice questions drill the exemption and recovery rules these systems depend on, and the full EPA 608 practice test covers the rest.

Frequently asked questions

What are normal pressures for R-290?

There is no single normal value, because pressure follows coil and condensing temperature. On the chart, a 20 °F evaporator corresponds to about 41 psig and a 100 °F condensing temperature to about 174 psig. Whether those are right depends on load and the manufacturer's data. On small sealed systems, confirm the charge by weight rather than by pressure alone.

What is the standing pressure of R-290?

About the saturation pressure at room temperature, as long as liquid remains: roughly 110 psig at 70 °F and 129 psig at 80 °F. It shows that propane is present but cannot show whether the charge is correct. A reading well below saturation for the ambient temperature means the liquid has leaked out.

How should R-290 be charged?

By weight, to the exact nameplate charge, using a scale accurate enough for charges measured in ounces or grams. Follow the manufacturer's service instructions for recovery, evacuation and charging, remove ignition sources, and use equipment rated for flammable refrigerants. Superheat and subcooling confirm the system is working but should not replace weighing in.

Is R-290 the same as grill propane?

It is the same molecule, but not the same product. Refrigerant-grade R-290 is high-purity propane made to a refrigerant specification. Fuel propane is sold to a looser specification and can contain other hydrocarbons and moisture, which a sealed refrigeration system does not tolerate. Only use refrigerant-grade R-290, and only in equipment designed for it.

Can you vent R-290?

Only in the end uses EPA lists at 40 CFR 82.154(a)(1)(viii), such as household refrigerators, stand-alone retail food cases, vending machines and self-contained ice machines. There, propane is an exempt substitute. Anywhere else it is non-exempt and the venting prohibition applies. Even where venting is legal, releasing propane near an ignition source is a fire hazard.

Does R-290 have temperature glide?

No. R-290 is a single compound, propane, so it boils and condenses at one temperature for each pressure. Its PT chart prints one saturation column, and a leak cannot change the composition of the remaining charge. Blends such as R-404A and R-448A do have glide, and their charts print separate bubble and dew values.

Sources

• National Refrigerants, R-290 Pressure-Temperature Chart
• Chemours, Pressure-Temperature Guide for A/C (OPTXLPTAC-2)
• Arkema, Forane 134a Pressure Temperature Chart
• 40 CFR 82.154, Prohibitions
• 40 CFR 82.156, Required practices
• EPA SNAP, Substitutes in Household Refrigerators and Freezers
• EPA, Section 608 Technician Certification Requirements